Transition from saturable absorption to reverse saturable absorption in MoTe2 nano-films with thickness and pump intensity

Abstract Nonlinear optical effect such as saturable absorption (SA) and reverse saturable absorption (RSA) play a key role for all-optical logic gates, in which transition between them is a challenging issue for the optoelectronic applications. Herein, we find MoTe 2 nano-films fabricated with liquid-phase exfoliation and a vacuum filtration method which demonstrate both SA and RSA with different thicknesses and pump intensity. The SA nonlinear optical susceptibility for thick film (100 nm) is Imχ (3)  ∼ 10 −11 esu. However, the thin film (30 nm) exhibited RSA with the Imχ (3) as high as ∼9.96 × 10 −11 esu. The different nonlinear optical response with the thickness is due to the interlayer coupling effect. Under high pump intensity, the thick MoTe 2 film also demonstrates transition from SA to RSA. The four energy-level model is employed to analyze the competition between the ground-state and excited-state absorption. We found that the conversion of SA to RSA depends not only on pump intensity but also on the absorption cross-section and transition probability between energy level. The transition properties of SA and RSA for MoTe 2 films can be used in all-optical logic gates, fast optical switch, optical limiter, mode storage and so on.